There is continuing demand for manufacturing processes that may lessen the cost, time or steps in producing a part. More often than not, the benefits resultantly associated with improving the manufacturing process are necessitated in the first instance by customer requirements to develop and improve products to have superior dimensional, mechanical and/or performance properties. For instance a typical differential side gear may have any or all of the following performance requirements such as: the spline area requiring dimensional precision, high shear strength and brinnelling resistance; the hub and thrust faces requiring dimensional precision, surface finish and case compatibility; the gear geometry requiring dimensional precision, surface finish and optimised profile; and the tooth and core strength may require impact resistance, wear resistance, spalling resistance, and different surface and core metallurgies. Different non-compatible manufacturing processes, i.e. casting, steel forging or powder metal forging, obtain different performance requirements for the same part, advantageously or otherwise.
Referring to FIG. 1, in order to meet some of these performance requirements, a gear 10 is made by forging a powder metal 14 and then case carburizing the gear to achieve a nearly constant effective case depth 16. The constant effective case depth 16 for each gear tooth 12 is shown in the partial cross-sectional view of FIG. 1. The parameters to be controlled to achieve nearly constant carburization of a fully dense part of specific hardness, case depth and carbon gradient are generally known. However, a nearly constant case depth does not necessarily achieve desired mechanical properties such as enhanced tooth wear and fatigue strength. It would be advantageous to achieve a better-controlled balance of these performance requirements in the final product, uncompromised by the manufacturing process thereby saving time, processing or cost.
A manufacturing process to improve the performance requirements of powder metal parts in a process known today as “sint-carb” is disclosed in U.S. Pat. No. 3,992,763 titled “Method of Making Powdered Metal Parts.” The process teaches carburization during or subsequent to sintering, and prior to forging, to enhance the case depth at a critical wall of the final forged product, eliminating the need for subsequent heat treating processes for achieving a case hardness. U.S. Pat. No. 4,002,471 titled “Method of Making a Through-Hardened Scale-Free Forged Powdered Metal Article Without Heat Treatment After Forging”, discloses a method of making forged powder iron base metal articles of high Rc hardness without need for further machining, surface treatment or heat treatment after quenching.
However, the above-mentioned patents do not teach or suggest any processes in which a variable case depth may be obtained in a final forged product to enhance performance features such as improved load bearing upon the tooth flank of a gear while providing impact resistance and bending fatigue resistance upon the tooth root.
Therefore, there is a need for a variable case depth powder metal gear exhibiting improved tooth wear resistance and load bearing at its flank surface, and improved impact resistance and bending fatigue in the tooth root. There is also a need for a method of manufacture of a variable case depth powder metal gear.